Disposable packaging containers, particularly those for storing liquids, are frequently produced from a packaging material which consists of a carcass layer of paper, which layer is covered with thermoplastic materials and aluminum foil. The packaging material is frequently supplied in the form of webs which are reeled on storage reels and which, after having been unrolled from their storage reel, are converted, by means of folding, into packaging containers in automatic packaging machines. A commonly occurring packaging of this type is that which is marketed under the trade name TETRA BRIK and which is preferably used for liquid filling material of the milk, fruit juice, etc. type.
This packaging container is produced in automatic packaging and filling machines in such a way that the web which is unrolled from the storage reel is transformed into a tube by the edges of the web being joined in an overlap seam, after which the tube which has been formed is filled with the intended filling material and subdivided into individual packaging containers by means of repeated transverse seals, which are arranged at a distance from each other and at right angles to the tube. After the filling material which has been supplied has been enclosed in this way in sealed-off parts of the tube, these parts are separated from the tube by means of cuts made in the said transverse sealing zones. The subdivided tube parts are then shaped, by means of folding along crease lines which are arranged in the packaging material, to form packaging containers of the desired shape, for example a parallelepiped shape.
Packagings of this type are frequently provided with opening notches in the form of holes, openings or slits which are made in the packaging material and which are covered with strips which can be torn off and which are normally termed “pull-tabs”. Alternatively the packaging device is provided with an external opening device, for example in the form of a plastic pouring spout having a screw top for resealing, which opening device is only allowed to penetrate the packaging laminate in connection with the packaging being opened and the product being used. In this connection, the packaging laminate is provided with an opening which consists of a punched-out hole in the carcass layer, over which hole aluminum foil and thermoplastic layers have been laminated. Thus, when the filling material consists of a sterile product, such as sterilized milk, or an acidic product, such as orange juice, the packaging container is frequently manufactured from a packaging laminate comprising an aluminum foil layer which makes the packaging extremely impervious to the penetration of gases, such as oxygen, which can oxidize the contents and impair its quality. In order to achieve the desired imperviousness, it is very important that the aluminum foil layer is not broken or damaged during the shaping of the packaging or when the packaging material is being manufactured and, for the function of the tear strip opening (the pull tab) or the penetrating opening device, it is of importance that the aluminum foil layer adheres extremely well to the area around the opening holes since otherwise the opening operation can easily fail. Thus when a covering strip has been affixed over the intended opening, this strip can be torn off in connection with this operation, without, for all that, the inner lining of plastic and aluminum foil being torn open. When a penetrating opening device is used, this opening device can fail to make a clean cut in the aluminum foil and thermoplastic layer, resulting in frayed edges.
An object of the invention is, in a simple and effective manner, to treat and prepare a packaging material web of the abovementioned type such that the edge of the packaging material web is effectively sealed off using an upper plastic film or a plastic film which is arranged around the edge zone. It is known to seal off liquid-absorbing material edges, which are exposed to the inside of a packaging container, with bridging thermoplastic strips or thermoplastic strips which are folded around the edges. It is also known, for the same purpose, to provide the packaging material web with what is termed a fixed plastic edge strip, i.e. with a plastic strip which projects from the edge of a carton web and which can be folded around the edge and sealed to its opposite side. Such a “fixed edge strip” is obtained by arranging carton webs alongside each other, such that they form a slit or gap between each other, after which the webs and the slits are jointly covered with a plastic foil or, in certain cases, with a plastic foil and an aluminum foil, after which the covered webs are separated by means of a cut made in the slit region, thereby forming a projecting, fixed strip. A drawback has hitherto been that it is not possible to obtain adherence, or it is at all events a poor adherence which is obtained, between, for example, an aluminum foil layer and a plastic layer in the region of the slits since the aluminum foil and the plastic layer cannot be pressed together within the region of the cut and, in particular, not right up to the boundary edges of the slits due to the varying thickness of the material and the resulting difficulty for the pressure rolls to compress the material within the slit region.
Packaging material of the type which is meant here can be produced by means of known technology, by applying the different layers, i.e. the aluminum foil layer, the inner plastic layer, etc., in several separate lamination operations, to the carcass layer consisting of paper or carton, and such a lamination procedure works very well when the carcass layer is not provided with holes, openings or slits, i.e. regions where the covering layers extend beyond or past edge zones of the carcass layer. It has been found that, when an aluminum foil is being layered onto a carcass layer web consisting of paper or carton, where the carcass layer is provided with holes, openings or slits, difficulties arise due to the fact that, in connection with the lamination, where the binding laminating layer frequently consists of a thin extruded thermoplastic film, the aluminum foil has to be pressed against the carcass layer base using an press roll or soft pressure roll in order to achieve sufficient adherence between the aluminum foil layer and the carcass layer. Since the aluminum foil layer is as a rule very thin (from approx. 5 to 10 μm), it comes to be pressed, by the press roll, against the edges around the opening or the slits in the carcass layer and partially pressed into the said holes or openings. Since the perforations have a relatively sharp edge, there is a risk of the aluminum foil rupturing and, at all events, the risk arises of the aluminum foil becoming creased around the edges of the openings or the slits and, as a result, either being weakened or achieving poor adherence with the carcass layer precisely in the edge zones of the holes or openings. Furthermore, the adherence between the plastic layer and the aluminum foil layer is poor in the region of the said holes or slits since the pressure of the press roll in the said regions is limited due to the reduction in the thickness of the material in the said holes or slits.
The abovementioned circumstances have constituted a severe problem which has firstly caused ruptures in the aluminum foil layer, and consequently caused the gas-tightness of the packagings to be inadequate, secondly caused the opening function to be defective due to poor adherence between the aluminum foil layer and the carcass layer in the edge zones around the openings and thirdly caused poor adherence between the aluminum foil layer and the plastic layer along the parts where the aluminum foil layer and the plastic layer project outside the carcass layer and are consequently not supported by this layer when being pressed together.
The adherence between the aluminum foil layer and the film of laminant thermoplast which binds the aluminum foil layer to the carcass layer, and the adherence between the aluminum foil layer and the plastic lining which is arranged outside this layer (i.e. the plastic lining which comes to be in direct contact with the liquid which is to be stored in the finished packaging) are especially critical. In the region of the said holes or slits, this adherence is frequently poor since the material exhibits a reduction in thickness in the holes or slits, which reduction in thickness results in the pressure of the press nip being lower at these sites. In other words, the reduction in thickness means that the press nip, comprising an press roll and a cooled roll, is unable to press the aluminum foil layer and the different polymer layers together sufficiently for achieving the requisite adherence in essentially the whole of the region defined by the hole or the slit, particularly in the vicinity of the hole edges. This is expressed as air inclusions adjacent to the edges of the hole or slit, which in turn means that problems of fracture formation in the aluminum foil arise, leading to impaired gas-tightness and consequently problems with asepsis. The air inclusions also result in it being difficult to tear off or penetrate the membrane consisting of the aluminum foil and the polymeric films in the hole/slit, with the ability to open the packaging being restricted and/or with it not being possible to make a clean cut when penetrating, resulting in the formation of frayed edges.
The difficulty of solving the problem of poor pressure in the press nip in the region of the holes/slits is aggravated by the fact that it is at the same time necessary, in the press nip, to pay regard to optimization aspects in relation to the adherence between the aluminum foil and the different polymer layers in the regions outside the regions of the holes/slits. The polymeric layer which is to be laminated to the aluminum foil is extruded in molten or semi-molten form directly into the press nip and has to be pressed together by the nip before the temperature of the polymeric material falls too much, in connection with which the material solidifies. This means that the line load in the nip is only effective for laminating during the course of a first, relatively short, press nip length. A press nip length which exceeds this effective press nip length only means that the line load is distributed over a greater area, something which is a disadvantage since the pressure in the press nip then becomes lower. For this reason, conventional press rolls are manufactured with a homogeneous surface facing of relatively high hardness, normally greater than 80-90 Shore A, giving a short press nip length. However, this relatively hard, homogeneous surface facing results in the adherence in the regions of the holes/slits being poor, as previously discussed.
Another problem in connection with laminating a polymer film to a carcass layer consisting of paper or carton, which carcass layer exhibits through holes, openings or slits, is that the polymeric material tends to accumulate in connection with these holes, openings or slits.
Problems with adherence between the different layers in the laminate can also arise in association with crease lines in the carcass layer consisting of paper or carton, with these problems being similar to those when holes, openings or slits are present.